An accumulating type (common rail type) fuel injector is known in which a fuel pumped from a high pressure feed pump is accumulated by an accumulator (so-called “common rail”), and then injected from a fuel injection nozzle into an engine cylinder at a predetermined timing.
In such an accumulator fuel injector, even when an engine speed is low, a predetermined fuel injection pressure can be maintained (the fuel injection pressure does not deteriorate). Accordingly, high pressure fuel injection contributes a great deal to an improvement of fuel consumption or high power of engine.
However, it is known that the smaller the nozzle injection opening diameter of a fuel injector, the more effective for realizing an excellent emission (for cleaning up exhaust gas). However, when an injection pressure by a conventional accumulator fuel injector (common rail injection system) uses a nozzle injection opening diameter which is smaller than an existing one, it is assumed that an injection period becomes too long at a high engine speed and at a high load region thereby hindering a high power of fuel injection.
Further, in recent years, an engine speed of a small diesel engine tends to be made higher. Here, an air speed in an engine cylinder increases substantially in proportion to an engine speed. For this reason, with the same injection pressure as before, during a high engine speed, spray is more likely to be run than during a low engine speed, air availability in a cylinder is deteriorated, and smokes (black smokes) are easily generated. Accordingly, in order to solve this problem, it is desired to further increase an injection pressure. However, an accumulator of the conventional accumulator fuel injector (common rail injection system) has a structure of usually accumulating therein a predetermined pressure (for example, in an existing common rail system, a maximum injection pressure is about 130 Mpa). In consideration of rigidity of the fuel injector, there is a limit in ability of further increasing the existing injection pressure (in other words, it is difficult to make a conventional injection pressure extremely higher).
On the other hand, among accumulator fuel injectors, a fuel injector further comprising an intensifier has been proposed (For example, Japanese Patent Application Laid-Open No. 8-21332).
The fuel injector according to this disclosure further comprises an intensifier for further increasing a pressurized fuel oil fed from an accumulator (common rail) by operating a piston operational switch valve. The intensifier comprises a pressure-increasing piston including a large diameter piston and a small diameter piston, and a plurality of oil channels connected to the piston operational switch valve. A fuel pumped from a fuel pressure pump is flown from the accumulator into the intensifier via the piston operational switch valve, and fed to both an injection control oil chamber (injector control chamber) for controlling an injection nozzle and the injection nozzle. When fuel is injected, a fuel injection control switch valve, which is provided at the injection control oil chamber, is controlled and switched to a low pressure injection for feeding fuel oil fed unchanged from the accumulator to the injection nozzle and injecting the fuel oil or a high pressure injection for feeding fuel oil further pressurized by the intensifier to the injection nozzle and injecting the fuel oil. Accordingly, a fuel injection mode suitable for an engine driving condition can be selected.
However, in this fuel injector, there have been drawbacks resulting in the following problems.
Namely, in the fuel injector, a fuel inlet area from the accumulator to the intensifier at a large diameter piston side and a fuel exit area of the intensifier at a small diameter piston connected to the piston operational switch valve is structured constant. Therefore, a time history of a fuel pressure during an operation of the intensifier is determined merely by a fuel pressure in the accumulator. These examples are shown in FIGS. 27A and 27B. As shown in FIG. 27A, if a transverse axis refers to time (second), a time history of a fuel pressure at a downstream side of the intensifier does not depend on an engine speed. On the other hand, as shown in FIG. 27B, if a transverse axis refers to an engine crank angle, the higher the engine speed, the slower the rise of the pressure. For this reason, particularly during a high load, as an engine speed becomes higher, there is no other choice than setting further longer an injection period at a crank angle base. Such an excessively long injection period can be a cause to hinder high power, which is not preferable.
In order to avoid this problem, a method can be considered in which, as the engine speed becomes higher, the fuel pressure of the accumulator (common rail) is increased, and power acting on the intensifier is increased to thereby increase a rising rate of a fuel pressure at a downstream side of the pressure-intensifying piston. However, at medium and high load regions, an injection pressure during a main injection must be high pressure, and yet, at this point, a pilot injection (fuel injection before the main injection) or a multi-injection (a plurality of fuel injections) is carried out for a purpose of reducing noise and improving exhaust gas. However, an optimal value of an injection pressure during the pilot injection is different from that of the main injection and generally lower than that of the main injection. This is because, since fuel is injected considerably earlier than a compression top dead point, air temperature or density in a cylinder becomes low, and when an injection pressure is set excessively high, complete penetration performance of injection becomes excessively large, thus resulting in a fuel deposition on a cylinder liner surface. However, in order to allow the proposed fuel injector to generate a high injection pressure at a region of a high engine speed, since it is necessary to increase a fuel pressure acing on a large diameter piston of the intensifier (fuel pressure of the accumulator), an injection pressure during the pilot injection, which injects a fuel of the accumulator unchanged, becomes higher and exceeds the optimal value. As a result, a fuel deposition on the cylinder liner surface cannot be prevented, which is estimated to be a cause to generate an incombustible HC or smokes.
Meanwhile, in an attempt to set a pilot injection (fuel pressure of the accumulator) appropriate for a high engine speed and a pressure-intensifying piston downstream pressure appropriate for operating the intensifier (for example, if a channel resistance is reduced by enlarging a fuel channel toward an intensifier large diameter piston), when the intensifier is operated during a low engine speed, a fuel pressure at a downstream side of the pressure-intensifying piston at a crank angle base rises steeply. This provides excessively high initial injection rate, increases a premixed combustion rate, and deteriorates NOx and noise. To avoid this, in an attempt to obtain an appropriate initial injection rate during the main injection by decreasing a fuel pressure of the accumulator during a low engine speed, an atomized state of the pilot injection injected by a fuel pressure of the accumulator is deteriorated, which can be a cause to generate smokes.
On the other hand, as shown in FIG. 28, for example, if the fuel injector is provided with characteristics in which a rising rate of a fuel pressure at a downstream side of the pressure-intensifying piston during the operation of the intensifier changes with time, even at a high engine speed or at a high load, in a state in which the pilot injection is set at an optimal fuel pressure, a high fuel pressure during the main injection (fuel pressure at a downstream side of the pressure-intensifying piston) can be obtained. By this, the above-described problems can be solved, and a low Nox, low noise, and high power engine can be realized. However, such settings were not conventionally achieved.
In addition, conventionally, a common rail with an intensifier (WO0055496) or an oil intensifier injection system and a pressure-intensifying injection system comprising an oil pressure and cam (DE4118237 and DE4118236) have been proposed. However, being different from the present invention recognizes an injection system behavior as a dynamic transient phenomenon, these disclosures recognize a period during which a pressure changes (inclination period of a pressure) as a transition period during which pressure changes from low pressure to high pressure. Accordingly, practical problems are caused on various controls or the like for intensifying pressure.
In view of the aforementioned facts, an object of the present invention is to obtain a fuel injection method in a fuel injector in which a fuel can be injected at a super high injection pressure which is much larger than that in a conventional fuel injector, a maximum injection pressure is not determined merely by a fuel pressure of an accumulator and is able to realize excellent combustible and exhaustive characteristics, and in which fuel injection can be performed with an arbitrary fuel injection pattern, whereby a degree of freedom of a fuel injection pattern can be further expanded (in other words, a maximum injection pressure, a rate of increase of an injection pressure at the start of intensifying pressure, a rate of decrease of the injection pressure at the completion of injection, a pilot injection pressure, and an after injection pressure of a fuel can be set freely).